Conventional ink jet printers using continuous jet printing feature ink-drop generating means that include an orifice plate connected to a vibrating means, and a waveguide member that supplies ink to the orifice plate and which guides a traveling wave along the orifice plate. Such printers also include a charge plate outside of the orifice plate, that charges appropriate ink drops ejected from the orifice plate. In such a printer, it is desirable that the orifice plate neither (a) transmits vibrations to the charge plate, nor (b) creates reflections in the traveling wave at the ends of the waveguide, which would tend to interfere with the acoustic energy created by the vibrating means.
Several previous methods for achieving these two functions are described in U.S. Pat. No. 4,110,759, issued on Aug. 29, 1978. The exterior space between the orifice plate and the charge plate is limited, since charged drops need to be charged while being detached from the jet emanating from the orifice plate. As a practical matter, the space between orifice plates and charge plates cannot exceed about 635 .mu.m. Thus, such previous methods have involved acoustic absorbers disposed within the waveguide. For example, elastomeric wedges have been formed within the ends of the waveguides, primarily to prevent reflections. Reflections are unwanted because they convert the traveling wave into a standing wave, which causes feedback to the generator and unequal energy levels. Although such acoustic absorbers have had some effectiveness, they are limited in that (a) their size is limited by the dimensions of the waveguide itself, and (b) by being exposed to the ink, the dampening means tends to deteriorate with age, particularly if formed from an elastomer. The latter problem is particularly acute when using solvent inks.
Therefore, there has been a need prior to this invention to improve upon the wave-dampening means heretofore commonly disposed within the waveguide that delivers the ink to the orifice plate.